A conventional jack employs hydraulic power to extend or retract the lifting piston which has a supporting member connected to the distal end thereof so as to engage with an object so that the object can be lifted by extending the lifting piston. The jack has a lever which is moved up and down to push the fluid in the paths in the jack to extend or retract the lifting piston. The lifting piston is extended very slowly because the volume of the fluid entering the paths of the jack is determined by the reciprocation of the lever. Each cycle of the lever makes the lifting piston move a certain distance so that the cylinder is moved at a fixed speed regardless of the load on the lifting piston. Therefore, even if the lifting piston is required to be extended or retracted in an emergency, there is no other way for the conventional jack to do this except by operating the lever step by step. This takes time and could result in a disaster.
The present invention intends to provide a jack whose lifting piston can be extended or retracted quickly by operating the lever so that the jack is helpful in practical use and saves time. The jack in accordance with the present invention has arisen to mitigate the disadvantage of the conventional jack.